Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals or channels, each having a different wavelength, are combined onto an optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the optical signal output from the laser, and an optical combiner to combine each of the modulated optical signals. Such components are typically provided at a transmit end of the WDM optical communication system to transmit the optical signals onto the optical fiber. At a receive end of the WDM optical communication system, the optical signals are often separated and converted to corresponding electrical signals that are then processed further.
Known WDM optical communication systems are capable of multiplexing 40 channels at 100 GHz spacing or 80 channels at 50 GHz spacing. These WDM optical communication systems occupy an overall bandwidth of 4000 GHz. At 50 GHz channel spacing and 100 GHz channel spacing, the occupied optical fiber bandwidth or spectrum is not efficiently used. As rapid growth of the Internet continues, and new applications arise, there is an increasing demand for higher data rates provided by underlying networks, which may be supported by advances in optical communication systems. Due to the increased demand, the information carrying capacity of an optical fiber preferably should also increase. As used herein, the terms “carrier”, “channel”, and “optical signal” may be used interchangeably.
One method to increase the data capacity of the occupied optical fiber bandwidth is to employ higher data rate modulation formats to modulate the optical signals or channels to carry data at higher rates. Such higher rate modulation formats, however, are typically more susceptible to noise, and, therefore, may not be used in transmission of optical signals over relatively long distances. Thus, the modulation format must be chosen according to a desired reach, or distance, the transmitted channels are expected to span. Other known systems, commonly called dense wavelength-division multiplexing systems (DWDM), are capable of packing even more densely, additional channels on an optical fiber by more closely spacing the channels together, such as at 25 GHz spacing between channels. While 25 GHz channel spacing is an improvement over 50 GHz and 100 GHz spacing, further improvement is still needed to meet the demands of increased data rates.
Conventional DWDM systems for optical communications typically conform to a wavelength or frequency grid defined by the International Telecommunications Union (ITU). The most common frequency grid is that used for channel spacing at wavelengths around 1550 nm as defined by ITU-T G.694.1 (2002). The ITU grid is defined relative to 193.1 THz and extends from 191.7 THz to 196.1 THz with 100 GHz periodic spacing between adjacent channels. Recently, however, as optical technology has improved, the grid has practically been extended to cover 186 THz to 201 THz and is sub-divided to provide the 50 GHz and 25 GHz spaced channels discussed above. Because the ITU grid is an accepted standard, many optical components used in known optical communication systems have been developed and optimized to conform to the ITU defined frequency channels and their periodic spacing. However, conforming to such a restrictive frequency grid, while convenient, may undesirably limit the data carrying capacity of an optical communication system.
Preferably, the information carrying capacity of an optical communication system should be optimized to carry a maximum amount of data over a maximum length of optical fiber while efficiently utilizing the bandwidth supported by available optical components, such as optical amplifiers, for example. Accordingly, individual carrier or channel spacing should be minimized according to the available optical components and transmitter and receiver technology capable of reliably transmitting and receiving such minimally spaced channels. Such minimum spacing may be less than 25 GHz, for example, and is preferably only slightly larger than the symbol rate of the modulation applied to each carrier. Therefore, the frequency difference between adjacent carriers is minimal and a greater number of channels or signals can be packed in a given bandwidth, resulting in more efficient use of network resources and the occupied optical spectrum of the channels. Accordingly, increased data demands of the network drive a need to provide a plurality of minimally spaced carriers to increase optical communication system network capacity.
The wavelength of an optical signal or carrier, however, can vary with temperature. Unless the wavelength of each optical signal is controlled, the wavelengths of the optical signals may drift and could equal one another, such that the optical signals interfere with one another. Alternatively, if wavelength changes vary significantly, the optical signal may not be filtered or demultiplexed at a receive end of the optical communication system. Wavelength control may be particularly difficult when the wavelengths are tightly or minimally spaced, such as when the optical signals collectively constitute a “superchannel.”
One known optical component that is commonly used to control optical signal wavelengths is a Fabry-Perot (FP) etalon, which may be used as a frequency discriminator to align an optical signal frequency or carrier frequency with one or more frequencies (i.e., a frequency “grid”) specified by a standard, such as the International Telecommunications Union (ITU). It is known that FP etalons exhibit periodic optical transmission characteristics, and that FP etalons are characterized by a free spectral range (FSR), or the distance in optical frequency between a pair of adjacent peaks in the transmission spectrum of the FP etalon. In known systems, an FP etalon is chosen with a FSR that matches the fixed frequency or channel spacing defined by the ITU grid, for example. However, in such systems the carrier spacing is typically 100 GHz, as discussed above. FP etalons have been developed with a FSR of 50 GHz or less, but to realize such an FP etalon, the physical thickness of the optical component is typically too large to be satisfactorily implemented. Additionally, FP etalons exhibit frequency errors. If such errors are small relative to the channel spacing, FP etalons may be used for wavelength control. However, if the carrier frequency spacing or carrier spacing is small, the frequency errors are a significant fraction of the carrier spacing. Accordingly, FPs may not be used to control wavelengths in systems having a narrow channel spacing, such as systems that transmit superchannels. As such, an improved method and apparatus for precisely locking the frequency or wavelength of an optical signal output from a laser, for example, is needed.